This invention relates to the manufacture of anhydrous alkali dithionites (hydrosulfites) by reacting an alkaline formate, an alkali metal agent, and sulfur dioxide in an alcohol/water solvent. It particularly relates to improving this process by producing a dust-free product.
In the process for the manufacture of alkali metal dithionites from an alkali metal salt of formic acid, an alkali metal hydroxide, carbonate or bicarbonate, and sulfur dioxide, the product precipitates in an alcohol/water solution. Upon completion of the dithionite reaction, the product is separated from the reaction filtrate, also termed the mother liquor, by filtration. The filter cake is washed with alcohol to remove the adhering filtrate, and the product is dried. The alcohol in both the filtrate and the wash alcohol is purified for re-use by distillation.
It is well known in the manufacture of dithionites that a portion of the dithionite product decomposes during the course of the reaction to form thiosulfate. Furthermore, this decomposition is autocatalytic with respect to thiosulfate; as the concentration of thiosulfate increases, so does its rate of formation.
It is also known that certain organic compounds are capable of reacting with or complexing thiosulfates. For example, U.S. Pat. No. 4,622,216 describes a method in which certain organic compounds are added during the course of producing dithionites to react with thiosulfate and thus minimize the decomposition reaction. These thiosulfate-reactive compounds include epoxy compounds such as ethylene oxide, propylene oxide, butyl and isobutyl oxide, epichlorohydrin, and epibromohydrin as well as halogenated hydrocarbons of the general formula RX or XRD, where R is an alkyl group of carbon number 1 to 8, or an allyl, methallyl, or ethylallyl group, and X is a halogen.
When such thiosulfate-reactive compounds are added to a batch reactor, they destroy thiosulfate ions as they are being formed within the reaction vessel and minimize destruction of the sodium dithionite product. The yield of anhydrous sodium dithionite is thereby increased. The disclosure of U.S. Pat. No. 4,622,216 is herein incorporated by reference. This invention relates to an improvement in the process of U.S. Pat. No. 4,622,216.
When sodium hydrosulfite (sodium dithionite) is produced from sodium formate, sulfur dioxide and an alkali method compound such as sodium hydroxide, the resulting dry product is obtained in high yield with a good purity and with excellent throughput and productivity. The product sodium hydrosulfite does contain, however, a variable quantity of small particles described as dust. While sodium hydrosulfite is not toxic, the dust is irritating and objectionable to those handling the material both at the manufacturing end and the user end. A certain amount of dust is typical of all sodium hydrosulfite made via any of the many sodium formate based processes as reported in the patent literature. (U.S. Pat. No. 3411875, 4126716, 4081521, 3917807, 3714340, 3897544, 4017593, 3826818, 3947559.) Microscopic examination of the sodium hydrosulfite particles reveals that, rather than being single crystals, they are agglomerates of a large number of much smaller, needle-like, individual crystals. Because of this configuration, the particles are relatively fragile. The many needle-like protrusions from the central mass are easily broken off, creating dust. The various manufacturing steps of filtering, drying, blending, conveying and packaging are performed in as gentle a manner as possible, yet result in a quantity of dust in the final packaged product that is somewhat variable, batch-to-batch.
No instrumental method of testing for "dustiness" has been found that correlates well with the "dustiness" perceived by a person handling the material. As a result, a means of quantifying the perceived dust has been devised and is described as follows:
1. Four standards varying from very little dust (No. 1) to very dusty (No. 4) were prepared and placed in glass jars with ample head space so that on inversion of the jar the suspended dust could be observed.
2. An identical quantity in an identical jar of a sample to be tested could then be inverted simultaneous with the various standards so that the perceived dust could be compared. In this way the sample could be rated on a numerical scale of from No. 1 to No. 4.
Both pilot plant and plant production batches are generally rated in the No. 2 to No. 4 dust categories, with a far lesser number of batches falling in the No. 1 category.
It has been found that the crystal habit of the product sodium dithionite is substantially modified by the addition of certain water soluble polymers to the reactor in which the sodium hydrosulfite is synthesized and crystallized. This crystal habit modification is such that the normally needle-like individual crystals become more-or-less cubical, and of a much larger size. Single crystals are still in the minority, but the bulk of the particles consist of a relatively small number of cubical crystals firmly bonded together. The resulting product is physically robust, and much less subject to mechanical attrition than the sodium hydrosulfite produced in the absence of the polymer. As a result, dust in the final product is minimal or non-existent; all products are rated as No. 1 or better with respect to dust.
While this discovery is unique to the manufacture of sodium dithionite via the formate process, it is not without precedent. Crystal habit modification owing to the addition of small quantities of a wide variety of substances to the mother liquor from which the crystals derive has long been known and practiced. There is a large body of literature--textbooks, journal articles, patents, and trade brochures--describing the results of such additions, but none shed light on why the phenomenon exists, nor give guidance towards selecting a material which will produce the desired result. The closest chemical to sodium hydrosulfite found in any of the literature is zinc dithionite crystallized via an evaporative process. U.S. Pat. No. 3,197,289 describes the use of a polyacrylamide of molecular weight 400,000 to 5,000,000, or gum resins of molecular weight 300,000 to 9,000,000, or nonionic polymers of ethers of cellulose of molecular weight 500,000 to 4,000,000 to alter the crystals of zinc dithionite. These polymers must be used in conjunction with glycerine and via a procedure specific to the production of zinc dithionite. None of the mentioned polymers were found to be of any benefit in reducing the dustiness of sodium dithionite. Not only does the crystal form of sodium dithionite differ from that of zinc hydrosulfite, but the conditions of crystallization in the respective manufacturing processes are so totally different that it would not be expected that crystal habit modifiers effective for the one would be effective for the other.
Various classes of compounds, other than polymers, suggested in the literature as being effective in specific cases were tested. These classes included surfactants, dispersants and salts other than sodium hydrosulfite. None shown any benefit.
Of all the water soluble polymers tested, one class, acrylics, showed excellent promise in both modifying the crystal habit and reducing the dustiness of the product. By acrylics is meant the polymers of acrylic acid, acrylamide, and various acrylates and methacrylates. In this class of polymers, those of molecular weight greater than about 200,000 are of no benefit. The greatest benefit is derived from those acrylic polymers of molecular weight less than about 60,000.
In using these acrylic polymers in the pilot plant, it was further discovered that the results achieved in reducing the dustiness of the product sodium hydrosulfite could be further improved by modification of the standard batch operating procedure. There are three major raw material streams to be fed to the sodium hydrosulfite synthesis reactor. These are a methanol solution of sulfur dioxide, a water solution of sodium hydroxide or other alkali, and a water solution of sodium formate. Past practice has been to start the sodium formate stream first, followed by a simultaneous start of the sodium hydroxide and sulfur dioxide streams. More consistently low dust values were achieved, however, when the sodium formate and sulfur dioxide streams were first started simultaneously, followed by initiation of the sodium hydroxide feed, or when the sodium formate stream was started first, followed sequentially by first the sulfur dioxide stream, then the sodium hydroxide stream. Either of these modes of operation produce temporarily a more acid condition within the reactor. It is believed that this greater acidity delays nucleation to some extent, and enables an essentially perfect repeatability from batch to batch in yielding a dustless sodium hydrosulfite product.
When this procedural change is involved without using one of the water soluble acrylic polymers, little or no benefit accrues to the product dust. Using an acrylic polymer without the procedural change results always in an improved dust number for the product, but the improvement is somewhat variable. The combination of the two, procedural change and addition of an acrylic polymer, results always in an essentially dust-free product.
The minimum quantity of the water soluble acrylic polymer that has been found necessary to achieve optimum results is 50 ppm based on the entire contents of the sodium dithionite synthesis reactor at the completion of a batch. Up to 200 ppm gives equally good results. Quantities less than 50 ppm are definitely of benefit, but the dustiness of the product increases as the concentration of polymer decreases below 50 ppm. The preferred concentration range is 50 to 200 ppm. Quantities in excess of 200 ppm may, of course, be used, but no additional benefit accrues to using this excessive amount, and there is an economic penalty.